13．Radioactivity 放射性
Radioactivity
放射性
    Atomic nuclei consist of combinations of protons, or positively-charged particles, and neutrons, or uncharged particles. 
原子核是由质子与中子相结合而构成;质子也叫带正电荷的粒子，中子也叫不带电荷的粒子。
The number of protons and neutrons in each element can vary, but only certain combinations are stable.
每一种元素中质子和中子的数目可能有所不同，而只有一些元素的组合比较稳定。
For example, calcium-48, having 20 protons and 28 neutrons, is a stable isotope of calcium.
例如钙-48含有20个质子和28个中子，是钙的稳定同位素。
But if there is an excess or deficiency of neutrons in any combination, the isotope will be unstable.
可是在任何一种组合中，如果中子数目不足或者过剩，同位素就不稳定。
A nucleus is more likely to be unstable if it is a heavy one — that is, if it contains a large number of protons and neutrons.
如果原子核是重核，也就是说原子核如含有大量质子和中子，就可能更加趋于不稳定。
Unstable nuclei attempt to achieve stability by emitting some form of radiation, until they transform themselves into stable isotopes.
不稳定的原子核总是要通过放射出某种放射线的方式达到稳定，也就是说直到原子本身衰变为稳定的同位素才能稳定。
    There are radioactive isotopes of every element, either those existing in nature or else those activated artificially by bombardment of stable nuclei with nuclear particles such as protons, alpha-particles or neutrons.
每种元素都有放射性同位素。有的放射性同位素是自然界中存在的，有的利用诸如质子、α粒子或中子一类的核子，对稳定的原子核进行轰击而用人工方法制造的。
However, a particle will not be absorbed by the target nucleus unless its velocity corresponds with one of the energy levels of the nucleus.
然而，除非粒子的速度与原子核的一个能级相适应，否则粒子就不可能为靶子核所吸收。
Heavy nuclei, having more energy levels than light nuclei, are more likely to effect capture of a particle — a fact which helps to explain the importance of uranium, thorium and other very heavy atoms in nuclear research.
重核比轻核具有更多的能级，因而可更有效地俘获粒子。这一事实有助于阐明铀、钍和其他重原子在核研究中的重要性。
    Since the neutron is uncharged, it is not affected by the charged electrons and protons of the target atom, and is therefore more likely to be captured than any other particle, provided that it is in resonance with an energy level of the nucleus.
由于中子不带电荷，所以不受带电粒子和靶原子的质子的影响。而且，如果中子同核的能级协调一致，就会更易于被其他任何粒子俘获。
In the event of neutron capture, the mass number of the nucleus will be raised, and it will thus become unstable and radioactive.
一旦俘获了中子，原子核的质量数便会增加，因而也就会变得不稳定并且具有放射性。
As radiation continues, the level of radioactivity falls exponentially, and the time taken for it to reach half its original value is known as the half-life of the isotope, which may vary from a fraction of a second to millions of years.
随着放射线的不断放出，放射性的能级呈指数式衰减，原子数衰减到原子原始数值的一半时所经历的时间称为同位素的半衰期。半衰期从零点几秒到几百万年各有不同。
Isotopes with long half-lives have many uses in medicine and industry, but they must be handled and disposed of with great care, in case they cause radiation damage.
半衰期长的同位素在医学和工业方面有许多用途，但必须极其慎重而严格地控制和处理，以免招致辐射的伤害。
    Neutron bombardment of the very heavy uranium atoms may have a quite different result.
用中子对非常重的铀原子进行轰击，可能会产生全然不同的结果。
It may cause the nucleus of the fissile U-235 atom to split into two parts.
轰击可以使可裂变的铀-235原子核裂成两部分。
This nuclear fission releases large quantities of energy which finally takes the form of the great energy, and at the same time other neutrons are ejected from the nucleus.
这种核裂变能释放出大量最终以热能形式出现的能量。与此同时，从核子里还放出另外一些中子。
The fission fragments are highly radioactive, and will contaminate the fissile uranium if they are not removed periodically.
分裂的裂片有着很强的放射性，而且如不定期清除，就会污染裂变的铀。
A number of these fission products, such as Caesium-137, are very useful as irradiation sources, and it is now possible to separate out the desired isotope from the spent fuel.
有一些裂变产物，例如铯-137,就是一种很有用的辐射源；目前已经能够从废弃的燃料中分离出我们所期望的同位素。
14．Chain Reaction 连锁反应
Chain Reaction
连锁反应
    When fission occurs, an average of 2-5 neutrons are emitted from the nucleus.
核发生裂变时，原子核平均会放射出2至5个中子。
If the fission process can be so arranged that one of these liberated neutrons is captured by another U-235 nucleus to produce another fission, then the reaction will become self-sustaining.
倘若所设计的裂变过程能使得所放出的一个中子被铀-235的另一个核子所俘获并进而产生另一裂变，那么随后反应就会自动持续进行。
    When emitted neutrons travel at a high velocity, and it is known that such fast neutrons have little chance of being captured by the fissile uranium.
中子被放射出时以高速运动。正如所知，这种高速运动的中子几乎不可能被可裂变铀俘获。
However, if slowed down to thermal speeds, their probability of capture is greatly increased.
然而，如能使中子减速到热运动速度，热中子被俘获的可能性就会大大增加。
In the normal thermal reactor, the uranium is surrounded by a large mass of moderating material.
在普通的热核反应堆中，铀被围在大量的减速材料当中。
The liberated neutrons collide repeatedly with the light atoms of the moderator in such a way that they lose much of their energy and eventually become thermalized.
释放出的中子与缓和剂的轻原子反复不断相互碰撞。碰撞的结果，中子便会失掉大量能量，于是最终会变成热速化中子。
The moderator may be either a liquid such as heavy water, or a solid such as graphite.
缓和剂可以是液体，如重水，也可以是固体，如石墨。
Both these substances are of low atomic weight and have low neutron absorption cross-sections.
这两种物质的原子量都低，而且中子吸收截面也低。
With the graphite moderator, the uranium which is generally in the form of rods is inserted into channels cut out of the graphite.
在使用石墨缓和剂的情况下，铀通常以铀棒形式插进石墨切槽内。
These channels are so arranged as to form a lattice structure, the object of which is to reduce neutron escape to a minimum.
这些切槽是以栅格结构排列的。这样做的目的是最大限度地减少逸出中子。
Provided that a sufficient mass of uranium is disposed in number of rods through the moderator, a high enough proportion of the emitted neutrons will find their way to fissile nuclei to produce a chain reaction.
假如质能充分的铀穿过缓和剂被配置在一些棒内，那么很大一部分放射出来的中子便有机会引发铀核裂变并进而产生连锁反应。
The minimum quantity of uranium required to initiate the chain reaction is called the critical mass.
引起连锁反应所需最小限度的铀量称为临界质量。
    Once irradiated, the uranium fuel elements tend to lose strength and become wrinkled.
经辐射，铀燃料的强度往往减弱并且会出现皱曲现象。
It is therefore necessary to encase them in a can or cladding of some materials such as aluminum or magnesium.
因此，需要将其装在密封容器内或者放在用某些材料如铝或镁做的护罩内。
These cans are designed so that they not only support the uranium inside, but also contain the highly radioactive fission products, and prevent reaction taking place between the fuel and the coolant.
设计这类容器或护罩时应使其不仅在内部能有效保护铀燃料，而且也可容纳放射性高的裂变产物，同时应能控制燃料和冷却剂间所发生的反应。
    A chain reaction can be initiated by inserting more and more fuel elements into the reactor core until the critical mass is attained.
只要在反应堆中加入越来越多的燃料并且一直加到临界质量为止，就可以发生连锁反应。
It can be terminated by withdrawing the rods.
把控制棒抽出来，连锁反应便终止。
reaction英语Once started, the chain reaction must be controlled in such a way that a steady neutron flux rate, and thus a steady production of heat energy, is maintained.
连锁反应一旦开始，就必须有效控制，以便保持稳定的中子通量率，进而保持稳定地产生热能。
The simplest method of control is by inserting control rods of cadmium, or some similar materials with a very high neutron absorption cross-section, into the moderator.
最简单的控制方法，是往缓和剂里插进用镉制的控制棒或加进用中子吸收截面高的类似材料制成的控制棒。
The purpose of the control rod is to absorb the neutrons emanating from a fissioned nucleus.
控制棒的作用是吸收由裂变核放射出的中子。
If therefore there is an increase in the neutron flux in the reactor, more control rods can be inserted until the reaction rate is stabilized again: that is, until the multiplication factor is exactly.
所以，如在反应堆内中子通量率增加，则放进的控制棒也就要更多些，而且一直要放到反应速率再度稳定为止，这就是说要一直放到再生指数正好是1为止。
15．Conductors and Conductivity 导体及传导性
Conductors and Conductivity
导体及传导性
    It is usual to consider electric current as a flow of electrons from one point to another through a medium, or even through a vacuum.
通常认为，电流是经介质甚至经真空而从一点流向另一点的电子流。
If the electron flow takes place in a vacuum, as in the case of electronic valves, the electrons will travel at considerable speeds, since little resistance is offered by the medium, and fewer impacts will occur between the electrons.
在电子管这样的真空中如发生电子流，电子便会以相当高的速度运动，这是因为介质产生的电阻极小，且电子间发生碰撞的现象也较小。
If the medium is a solid — in which case the electrons are more tightly packed — the electron flow will be slower.
如果介质是固体，电子便敛集得较紧密，因而电子流就流动得慢些。
    All substances may be classified electrically as conductors or insulators, according to the degree of resistance which the medium offers to the flow of current.
一切物质都可根据其介质对电流产生的阻力不同而在电学上将其分类为导体和绝缘体。
Most liquids, particularly solution in liquids, are good conductors.
大多数液体，特别是溶液，都是良导体。
Most gases at normal temperature and pressure are good insulators, but gases maintained at low pressure in a sealed tube allow a flow of current to take place as a result of ionization of the gas molecules, ？？？while others are so resistant that they are referred to as insulators.
常温常压下，大部分气体是好的绝缘体。不过，通过气体分子的电离作用，密封管中的低压气体可使电流流动，？？？而其它一些电阻很大以致可称其为绝缘体。
Electric current is normally transmitted along *annealed [退火] copper wire.
电流通常沿着软铜线传导。
    The resistance of any material to the flow of current is affected by a number of factors, such as the length and cross-section of the conductor, and by its resistivity, which is a specific property of the material at a specific temperature.
任何材料对电流的阻力都受到很多因素的影响，例如导体的长度和截面积以及导体的导电率。导电率是材料在特定温度下特有的特性。
The temperature therefore also has some effect on the resistance of a material: in most cases, an increase in temperature causes an increase in resistance.
因此，温度对材料的电阻也有一些影响。在大多数情况下，温度升高必然引起电阻增大。
With certain metals, such as copper or iron, the change in resistance which attends on changes in temperature is relatively large — a fact which is utilized in the resistance thermometer, in which it is possible to measure temperature changes, as in the *windings [线圈] of an electric motor, for instance, by the change in resistance.
某些金属如铜和铁，随着温度的变化，电阻的变化也相当大。电阻变化的这一规律被应用在电阻温度计中。例如，电阻温度计就能够依靠电阻的变化来测定诸如电机线圈的温度变化。
    Some materials have a very high resistance, and as such they can be used as insulators to prevent the leakage of current.
某些材料的电阻很高，因此可用作防止漏电的绝缘体。
Among these materials are asbestos [石棉], *celluloid [赛璐珞], porcelain, cotton and rubber, and recently a number of new materials have been developed, including synthetic textiles such as nylon, and *synthetic resins [合成树脂] such as vinyl [乙烯基] resins.
这些材料是石棉、赛璐珞、陶瓷、棉花以及橡胶；另外还有近代研制出的许多新材料，其中包括尼龙一类的合成纤维织物及乙烯基树脂一类的合成树脂。
The resistivity of most insulators decreases with an increase in temperature, for which reason the temperatures in insulated conductors must be kept reasonably low.
大多数绝缘体的电阻率随着温度升高而有所降低。为此，绝缘导体的温度一定要保持适当低些。
A breakdown of insulation may occur under the application of very high voltages, and it is necessary to know the *dielectric [非传导性的] strength of any insulating material.
电压非常高时，绝缘体可能被击穿。这就需要了解绝缘材料的绝缘强度。
Some materials, such as cotton, which is often used as insulation, are liable to absorb moisture, and this will adversely affect their insulating properties.
某些材料比如常作为绝缘物质使用的棉花，比较易于吸湿。这会对绝缘性能有不良影响。
Rubber, which is a standard insulating material, is liable to deteriorate under sunlight, and it is therefore advisable to protect it with some weatherproof material.
橡胶是标准的绝缘材料，但在阳光下易老化变质，因而需应用某些防老化剂，以防止橡胶老化。
16．Liquid Pumps 液泵
Liquid Pumps
液泵
    A wide variety of liquids are now being used in chemical plants, and these usually have to be pumped through pipelines.
当今化工厂大量使用各种液体，这些液体通常都要通过管路用泵输送。
In choosing the type of pump most suitable for any specific operation, a number of problems have to be taken into account.
选择足以适应各种专用要求的泵型时，应考虑到许多因素。
In the first place the quantity of liquid, and the pressure at which it is required to be pumped, must be considered.
首先，应当考虑到液体的数量以及泵送这些液体所需要的压力。
Neglecting all other considerations, the reciprocating pump is ideal for pumping small quantities of liquid at high pressure, the amount of fluid delivered depending on the volumetric displacement of the pistons.
如果不考虑所有其他因素，往复泵是高压下泵送少量液体的理想机械；所输送的液体取决于活塞的排气量。
The delivery however is rather uneven — a defect which can only be remedied by compounding a number of cylinders, thus making the machine rather large and expensive.
然而这种往复泵的输送流量相当不均匀，这个缺点只能通过配置多个汽缸加以解决。这样做泵体必然笨重，费用会增高。
    The viscosity of the liquid is another factor, which must be taken into account, in that it largely determines the frictional losses, which will occur.
    液体的粘度是另一个应考虑到的因素。根据液体的粘度，基本上确定出液体会产生的磨擦损失量。
Rotary-type pumps are widely used in the handling of highly viscous liquids.
处理高粘度液体，多用旋转液体泵。
They differ from reciprocating pumps in that they deliver an even flow of liquid, but they are unsuitable for pumping liquids of low viscosity, which tend to leak past the tips of the gear teeth.
这种泵与往复泵的不同点，在于所输送的液体流量均匀，但不适于用来输送低粘度的液体，因为低粘度液体流经轮齿定端时易发生渗漏现象。
They are mainly used for the pumping of oils and similar liquids of high viscosity, which are less liable to leakage and which moreover provide the necessary lubrication for the moving parts of the pump, thus obviating the need for a separate lubricant.
旋转泵大都用来唧送石油或类似的高粘度液体。这是因为这种液体不易渗漏，且又可为泵的转动部分提供必要的润滑。这样就无须另加润滑剂。
A further consideration involved in the choice of pump is whether or not the liquid is corrosive or contains solid particles in suspension.
选择液体输送泵时，还要进一步考虑到液体是否具有腐蚀性，是否含有悬浮的固体颗粒。
In such cases, precautions have to be taken to avoid damage to the mechanism.
如有，就必须采取措施，防止损坏。
With regard to suspensions, the clearances in the pump must be large enough to permit the particles to pass, and from this point of view, the rotary pump is not suitable, clearances necessarily being small to reduce leakage.
在有悬浮体的情况下，泵的间歇必须适当大些，以使悬浮颗粒得以通过。就这一点来说，旋转泵却不适用，因为要减少渗漏，泵的间隙必须很小。
The centrifugal type of pump is more commonly used when suspensions are present, since various types of impellers can be fitted, thus enabling the pump to handle a wide variety of liquids, including those with suspensions.
液体含有悬浮颗粒时，一般常常使用离心泵。这是因为离心泵可装各种叶轮，因而使离心泵能够处理各种液体，其中包括那些含有悬浮颗粒的液体。
    The centrifugal pump is compact and requires little maintenance.
        离心泵体积紧凑，很少需要维修。
But it suffers from a certain disadvantage, in that there are considerable friction losses in the entry and discharged passages of the impeller, and further losses due to turbulence in the impeller itself.
但离心泵也有一定的缺陷：主要表现在入口处及叶轮喷出流道处有较大的摩擦损耗；以及因叶片本身的湍流所引起的另外一些损耗。
The pump consists essentially of one or more impellers rotating in the center of a casing.
离心泵基本上是由泵体中心旋转的一个或多个叶轮组成。
This impeller contains a number of vanes so designed that the fluid entering the pump is carried round by the vanes at a speed depending on their speed of rotation.
叶轮上装有许多叶片，所设计的那些叶片足已使进入泵内的流体按照叶片转速的速度，由叶片旋转送走。
It is then discharged into a delivery chamber with a high kinetic energy imparted by the action of the vanes.
然后，在叶片的较大动能的作用下，液体被排进一个压液室。
This energy is then converted into pressure energy, thus adding to the large pressure difference between the suction and delivery sides of the pump.
随后这种动能便转换成静压能，于是又加大了泵的吸入管与排出管间本来就很大的压差。
Excessive speed of rotation of the impeller is liable to cause the pressure on the suction side to fall so low that the liquid will vaporize, thus causing damage to the impeller.
叶轮转速过大，易引起吸入压力降低，甚而导致液体汽化，因而会损坏叶轮。
17．Road Foundations 道路基础
Road Foundations
                          道路基础
    In planning a road, extensive preliminary surveys must be carried out to determine the precise line of the road, and to work out how much earth will require to be moved and what quantities of surfacing material will be needed. A second purpose of the surveys will be to take samples of the different soils encountered at different depths by boring, in order to decide whether they are suitable for use or whether they must be replaced by imported fill. This is of great importance, since various types of soils have properties which result in low bearing capacities.
    设计道路时,应进行初步而广泛的勘测工作,以确定道路的精确基线,算出需掘除的土方以及所需路面材料的数量。勘测工作的另一目的，是对不同钻探深度所遇不同土质进行取样，从而确定各层土质是否适用或者是否需要换以回填土。这样做很有必要，因为土质纷杂，必然会降低路面承载能力。
    Failure in road surfaces is usually attributable to insufficient preparation and compaction of the sub-grade — that is, the soil on which the surface of the road is laid. Certain soils, such as clay or peat, are unstable, either because they are largely impermeable and hence difficult to drain or because they cannot be properly compacted. It is sometimes possible to stabilize some soils with cement, but in most cases it will be necessary to excavate the soil to a considerable depth and to replace it by a suitable granular soil. The most stable sub-grade soils are gravel and sand, both being readily compactable and easy to drain. It is often unnecessary to excavate these soils to a depth of more than three or four inches, and, if sufficient supplies are available they can be used as filling material, particularly on embankments, where the soil must be capable of a high degree of compaction.
    上面铺敷路面的那层土壤即路基，铺筑和夯压得不合要求，往往是路面破坏的原因。某些土壤如粘土或砂质土是不够稳定的土壤。这一方面是因为这类土壤渗透性极度差，因为难于排水；另一方面是因为对这类土层不能有效地进行夯压。有时，可用硬化剂对某些土壤进行稳定硬化处理。不过在大多数情况下需要将土层挖到适当深度，再用颗粒适宜的土壤进行更换。最稳定的路基土壤是砾石土壤或砂质土，这两种土壤既适于夯压又利于排水。这类土层的挖掘深度通常不适宜于超过3至4英寸，而且这类土壤土源充足，还可作为回填土料使用，特别是在筑堤工程中可用作填充土，因为在筑堤工程中更需要能以有效进行夯压的土壤。
    The stability of a soil is largely dependent on an unchanging moisture-content, and to assist this, adequate drainage is necessary, although in the case of heavy clays no form of drainage is very effective.
  土壤的稳定性能在很大程度上取决于稳定的潜藏含水率；而要确定这一点，即使在没有更为有效排水方式可取的重质粘土的情况下，也还是要有适切的排水措施。
    Mechanical excavation is carried out by a variety of machines, including the shovel and drag-line excavator. The choice of plant used will depend on how deep a cut is required and also on how accessible the cut is. After the soil has been excavated to the appropriate depth and filled, it is compacted by a roller until it is firm. Following this, it is common practice to lay a sub-base over the sub-grade soil in order to strengthen it, and to ensure that the traffic load shall be distributed as widely as possible over the foundations. The sub-base is normally composed of granular material with good drainage characteristics, and will vary in depth according to the nature of the sub-grade, and also according to what thickness of concrete is to be laid above it.
    可用各种机械进行挖掘，其中包括翻斗挖土机和拉铲式挖土机。应根据所需的挖掘深度和可用的挖掘方式不同选取所需用的机械设备。土壤挖到适当深度且以回填以后，可用压道机滚压，直到路基压实为止。而后，为增强路基强度，常用的技术措施是，在路基土层上面铺敷一层底基，并且尽可能做到使通行载荷充分地分布在整个基座上。一般说来，底基也用具有良好排水性能的颗粒土壤构筑，并且随着路基的性质和底基上面应铺混凝土厚度的不同，底基的厚度也有所不同。
    It is essential that the sub-base should be compacted to a uniform density, since the density of a soil is closely related to its bearing capacity. The compacted soil is then covered either with a sealing coat of tar, or with rolls of waterproof paper, the object of which is to prevent liquid cement from the concrete base from seeping into it, thus weakening the lower layers of the concrete and increasing the moisture content of the base.
  底基应压实得密度均匀，因为土壤的密度直接影响土壤的承载能力。压实完的土层，随后要涂以沥青封闭层或铺以防水纸卷材。这样做的目的，在于防止混凝土底基的水泥浆渗入夯压土层，进而可防止混凝土底层的强度遭到削弱并可防止基座含水量增加。

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